This invention relates to acoustical couplers adapted to communicate through ordinary telephone handsets computer or other data from a modem, and particularly relates to compensation techniques therefor.
It is a standard technique for a central computer or other data center to have one or more terminals at a location remote from that of the data source. The use of existing telephone circuits is often the most convenient and least expensive way of communicating between the data center and the remote terminals. A telephone circuit is connected to the data source at the data center through a modem which conditions and translates the computer digital data into a form which can be carried by ordinary telephone lines, given their limitations. A matching modem is utilized at each remote terminal.
One such modem type is manufactured and sold by the Racal-Vadic Corporation, their number VA 3400, which is the subject matter of U.S. Pat. No. 3,937,882--Bingham (1976). Such a modem for the terminal locations is adapted for transmitting and receiving information from the telephone line at two distinct frequency bands, so called upper and lower bands. The upper frequency band is used for passing data signals in one direction and the lower frequency band is used for passing data signals in the other direction. This freqency band division with quadrature amplitude modulation and dibit phase encoding helps achieve 1200 bps. full duplex operation on the telephone lines. The modems themselves are designed for direct electrical connection to the telephone line and contain circuitry to compensate for the limitations and nonlinearities of the telephone line.
It is often desirable to use at the terminal location an acoustical coupler wherein the modem signal is coupled to the telephone line through an ordinary handset of a telephone, rather than being electrically wired to the telephone line. This is accomplished by providing an acoustical coupler with a microphone and speaker for resting against, respectively, the receiver and transmitter of the telephone handset. This acoustic coupling introduces further nonlinearities in the data transmission path and it is a principal object of the present invention to provide an electronic circuit to compensate for such nonlinearities in order to have a reliable acoustic coupler compatible with modems using QAM techniques.